You need to buy a MP4,1 backplane or a MP4,1 backplane already upgraded to MP5,1 firmware.
A real MP5,1 backplane won't work with your tray since MP5,1 SMC firmware is 1.39f11 while MP4,1 SMC firmware is 1.39f5. Mismatched SMC between trays and backplanes makes all the case fans work at full RPM, full time.
The real problem is with Westmere Xeons, if you already upgraded your tray to one/two in the past. If you buy a MP4,1 backplane that was never flashed with MP5,1 firmware, you have to downgrade your Xeon(s) from Westmere back to Nehalem and then flash the BootROM to MP5,1. If you don't do the MP4,1 to MP5,1 flashing process, you can't use a Westmere Xeon since only MP5,1 firmwares support Westmere Xeons.
Yes.
CPU trays don't have BootROM firmware, just SMC firmware.
MP4,1 SMC firmware is not user or technician updatable, only Apple can update/downgrade SMC firmwares since it's a secure micro controller that needs a key to be updated, this key never leaved Apple possession. It's one of the few things that never leaked, right next to the MP4,1/MP5,1 electrical schematic.